The present invention is directed to apparatus for measuring a predetermined characteristic of a material in sheet or bulk form using two or more wavelengths of radiation; and more particularly, material moving in a pipeline such as paper pulp where a ratio of received radiation of two wavelengths provides a measurement.
In the paper making field, the measurement of the paper material by a ratio measurement is well known. For example, one wavelength is utilized as a reference which is relatively unaffected by the characteristic being measured and the other is the actual sample measurement. Naturally, the choice of these two wavelengths or wavelength bands is critical. Normally the best choice for a reference band is as close as possible to the measurement of sample band. In order to separate both the reference and sample wavelengths efficiently, a beam splitter has been used. This is unsatisfactory in many applications since the cutoff frequencies of the beam splitter are not sharp. This is especially troublesome where the wavelengths of interest are close to each other. In the paper making art, for example, critical wavelengths for reference and sample wavelengths for the measurement of moisture are in the range of 1.7 to 1.9 microns and known beam splitters are difficult to manufacture in this wavelength range. In addition if more than two wavelengths are used, beam splitters are very cumbersome. They are also critical with regard to sensitivity contours.
Another prior art technique for receiving and separating multiwavelength radiation for the purpose of measurement is the use of a diffusing plate to spread radiation evenly to several detectors. Here the radiation from plate to detectors is, by design, diffused, arriving at a wide variety of angles. Only absorbing body type filters can be used in this arrangement which are limited in bandwidth and band center characteristics. The versatile interference narrow bandpass type filters require nearly parallel radiation. This, of course, is not provided by the diffusing plate type apparatus above.
Where it is desired to utilize a single sided gauge for the measurement of moisture in moving sheet material, the use of a single detector with spinning filter wheels for the 1.7 and 1.9 micron wavelengths has been suggested. Because of the necessary time lapse the single detector is actually looking at two different portions of the moving material. If two detectors with a beam splitter are used, the splitter is sensitive to the flutter of the paper and other spatial effects.